1. Field of the Invention
The present invention relates to an electrical switching apparatus operating mechanism and, more specifically to a closing protection mechanism for a closing assembly having an over-toggle linkage.
2. Background Information
An electrical switching apparatus, typically, includes a housing, at least one bus assembly having a pair of contacts, a trip device, and an operating mechanism. The housing assembly is structured to insulate and enclose the other components. The at least one pair of contacts include a fixed contact and a movable contact and typically include multiple pairs of fixed and movable contacts. Each contact is coupled to, and in electrical communication with, a conductive bus that is further coupled to, and in electrical communication with, a line or a load. A trip device is structured to detect an over current condition and to actuate the operating mechanism. An operating mechanism is structured to both open the contacts, either manually or following actuation by the trip device, and close the contacts.
That is, the operating mechanism includes both a closing assembly and an opening assembly, which may have common elements, that are structured to move the movable contact between a first, open position, wherein the contacts are separated, and a second, closed position, wherein the contacts are coupled and in electrical communication. The operating mechanism includes a rotatable pole shaft that is coupled to the movable contact and structured to move each movable contact between the closed position and the open position. Elements of both the closing assembly and the opening assembly are coupled to the pole shaft so as to effect the closing and opening of the contacts. The closing assembly may be actuated manually by a user input or in response to an input from a remote actuator.
The trip device included an over-current sensor, a latch assembly and may have included one or more additional links that were coupled to the toggle assembly. Alternately, the latch assembly was directly coupled to the toggle assembly. When an over-current situation occurred, the latch assembly was released allowing the opening spring to cause the toggle assembly to collapse. When the toggle assembly collapsed, the toggle assembly link coupled to the pole shaft caused the pole shaft to rotate and thereby move the movable contacts into the open position.
Low and medium voltage electrical switching apparatus typically had stored energy devices, such as a closing spring and an opening spring, and at least one link coupled to the pole shaft. The at least one link, typically, included two links that acted cooperatively as a toggle assembly and which were coupled to each other at a toggle joint. When the contacts were open, the toggle assembly was in a first, collapsed configuration and, conversely, when the contacts were closed, the toggle assembly was, typically, in a second, toggle position, that is, an in-line configuration, or in a slightly over-toggle position. The closing spring was usually compressed, or “charged,” by a motor or a user utilizing a lever arm. The closing spring, typically, holds more stored energy than the opening springs and during the closing operation wherein the contacts are moved to the second, closed position, the opening spring was charged. The opening spring biased the pole shaft, and therefore the toggle assembly, to the collapsed position. The opening spring and toggle assembly were maintained in the second, toggle position by the trip device.
When the contacts were in the first, open position, the toggle assembly links, which define lines of force, were “folded,” typically at an acute angle. When the mechanism was closing, a closing component applied a closing force to the toggle joint. The closing component moved the links until the lines of force, that is, the links, were nearly in-line or on “center.” If the fully closed position of the separable contacts was reached before the lines of force were fully in-line, the closing assembly is an “under-toggle” mechanism and the toggle joint continued to rest on the closing component to prevent the toggle joint from collapsing. In this type of closing assembly, the closing component was, typically, a cam. If, during closing, the closing component moved the toggle joint through the in-line position and beyond, the closing assembly is an “over-toggle” mechanism and the toggle joint typically rested upon a stop that is separate from the closing component. That is, the toggle joint typically came to rest on a stop pin that prevented the toggle joint from collapsing in a reverse direction.
In either an under-toggle or over-toggle mechanism, the contacts would initially engage each other when the angle of the lines of force were approaching the in-line position. After the contacts engage, the driving force required to complete the closing of the contacts increases. That is, prior to the contacts engaging each other, the closing component was, essentially, only moving the moving contact and compressing the opening springs. Once the contacts engaged each other, the closing component was required to overcome any electromagnetic forces generated by a current passing through the contacts, as well as, forces created by the contact spring as they were being compressed. If the closing component was not able to overcome these forces, there was a chance that the closing operation could stall. If the closing operation stalls, dangerous arcing may occur at the contacts if the contacts are subject to inadequate force or support, for example is the contacts are held in close proximity or if the contacts slowly separate from each other.
Some under-toggle mechanisms have attributes that mitigate the consequences of a stall. That is, when the closing component is a cam acting upon the toggle joint, the cam surface is rising, that is, increasing in radius, so as to effect the movement of the toggle joint. Such a cam is structured to rotate in a single direction during closing, wherein the radius of the cam is increasing, and subsequent charging, wherein the radius of the cam is generally constant. Thus, if a stall occurs, the cam needs only to be rotated further, such as by charging after the close attempt, to cause the toggle joint to be moved into the proper position.
An over-toggle mechanism, however, is not structured to be supported by the closing component. Typically, the closing component acts upon the toggle joint and is then, slowly, withdrawn during the charging of the closing spring. Thus, unlike an under-toggle mechanism, a stall in such a closing assembly could allow the toggle joint to return to the open configuration. If, for example, the toggle joint is resting on the closing component as it is being slowly withdrawn, the contacts will be slowly separated allowing for dangerous arcing to occur.
It is further noted that a device may have a high-current capacity for withstanding an electrical fault that appears after the device is already closed, but may not have enough mechanical energy to complete a closure on that same fault current. That is, high current flowing in the device adds electromagnetic force to the springs which resist closing and increasing the mechanical energy to close on all such faults would shorten the mechanical life or add cost to the mechanism. The trip device is often self-powered by current passing through the contacts of the electrical switching apparatus, and therefore the trip device is inactive before closing. If a fault current which is higher than the closing, or “making” capacity, but lower than the “withstand” capacity appears in the electrical switching apparatus, the trip device must determine if the operating mechanism is closing, in which case the trip device should trip open to protect against harmful arcing at the contacts due to stalling at less-than-fully-closed, or the operating mechanism was already closed, in which case the trip device should remain closed until the manufacturer or customer-programmed delay time for tripping is reached.
One strategy for immediately tripping an operating mechanism that is closing on a fault above its making capacity is the use of a “time-delay” switch. This type of switch senses the state of the device, typically by sensing the pole shaft position, and connects to the trip device. The switch is held in one state when the device is open, and released to move to its other state when the electrical switching apparatus is closed. The switch assembly typically contains a mass with a relatively light bias spring resulting in an inertial delay off its motion when the device closes. This delay serves as a mechanical memory used by the trip device when a fault current above the making capacity appears. If the switch indicates the “device-closed” position, then the device was already closed some moments before the current appeared and the operating mechanism is not attempting to close on the high current; therefore it is not necessary to trip open to protect against prolonged harmful arcing. If the switch still indicates the “device-open” position, then the device was open moments before and the current flowing is the result of a closure attempt. Thus, the trip device must immediately re-open the contacts to protect against a potential stall.
As a result of its kinematics, an over-toggle mechanism has the characteristic of “over-driving” the contacts as the lines of force passes through in-line, or “center”, before settling back to the full closed position. Therefore, in a normal closing, the pole shaft is at the full closed position twice; once before the lines of force reach center, and again after passing through center. A switch sensing the pole shaft position, such as the time delay switch, is not able to discriminate between fully closed and partially-closed, where it could potentially stall. Despite these characteristics, there are some reasons to select over-toggle mechanism for some applications, rather than under-toggle mechanisms.